Hitherto, radiation images such as X-ray images have been widely applied for diagnosing in medical field. Particularly, an intensifying screen-film radiation image forming system is still used in the medical field in the world as an image forming system having high reliance and superior cost performance as a result of improvement in sensitivity and image quality in long history of the system. However, obtained in this system are analog data which are not suitable for easy image processing nor quick image transfer.
After that, as a digital radiography system, computed radiography (CR) has been developed. In the CR system, a digital radiographic image is directly obtained and can be displayed on a display device such as a cathode ray tube or a liquid crystal panel, whereby image formation on a photographic film has become unnecessary, and the convenience of diagnosis in hospitals or in medical clinics has become largely improved.
The CR system has been mainly adopted in the medical field where X-ray radiographic images have been obtained using stimulable phosphor plates. Herein, “stimulable phosphor plate” has a structure in which a layer of stimulable phosphor is formed on a substrate and it has a function of (i) storing the radiation passed through an object; and (ii) releasing the stored energy as stimulated light emission of which intensity depends on the dose of stored radiation, by being irradiated with an electromagnetic wave (stimulating light) such as infrared rays to sequentially excite the stimulable phosphor.
However, the stimulable phosphor plate has not been fully satisfactory in providing an enough S/N ratio (Signal to Noise ratio), an enough visibility nor an enough spatial resolution of the image and the image quality has not been as high as that of the intensifying screen-film system. As a new digital image forming technology, Flat Panel Detecter (FPD) utilizing thin film transistors (TFT) has been developed, which has been reported, for example, in “Amorphous Semiconductor Usher in Digital X-ray Imaging”, John Rowlands et al., Physics Today (November, 1997), p 24, or in “Development of a High Resolution, Active Matrix, Flat-Panel Imager with Enhanced Fill Factor”, L. E. Antonuk, SPIE (1997), Vol. 32, P2. The FPD enables to obtain radiographic images as digital information and enables to freely process or quickly transfer the digital image data.
Also, the FPD has advantages in that it is smaller in size and has superior properties in displaying moving pictures when compared with the CR system. However, the same as in the CR system, the image quality of FPD has not fully been as high as that of the intensifying screen-film system and further improvement has been desired.
In FPD, utilized is a scintillator plate having an X-ray sensitive phosphor to convert the radiation to visible rays, however, the S/N ratio of the scintillator has not fully been large enough due to a considerably large electric noise generated from the TFTs or from the electric circuits to drive the TFTs, when the dose of radiation is low, resulting in failing to provide an enough emitting efficiency to attain high quality images.
The light emission efficiency generally depends on the thickness of the phosphor layer, however, when the thickness of the phosphor layer increases, the sharpness of the image by the scintillator plate is lowered due to the scattering of emitted light in the phosphor layer.
Consequently, investigation of materials has proceeded by which the lowering in sharpness can be inhibited while sufficiently keeping the thickness of the phosphor layer. As a result of that, cesium iodide has been utilized. Cesium iodide exhibits high efficiency of conversion from radiation to visible light and the columnar crystals thereof can be easily formed by vacuum evaporation. Therefore, scatter of the emitted light in the crystal can be inhibited by the light guiding effect of the columnar crystals even when the phosphor layer is made thick.
When a phosphor layer containing CsI is formed, various activators are used because the emission efficiency is not enough when CsI is used alone, and when the content of an activator is 0.01 mol % or more based on CsI (based on the mole of CsI) base material, improved emission efficiency is obtained.
For example, in Patent Document 1, disclosed is a technique in which an X-ray sensitive phosphor exhibiting an improved radiation-visible ray conversion efficiency is obtained by vacuum evaporating a mixture of CsI and sodium iodide (NaI) in an arbitrary mixing ratio onto a substrate to deposit sodium activated cesium iodide (CsI:NaI) crystals, followed by annealing.
Recently, a technique to prepare an X-ray sensitive phosphor has been disclosed, for example, in Patent Document 2, in which CsI is deposited via vacuum evaporation and an activator, for example, indium (In), thallium (Th), lithium (Li), potassium (K), rubidium (Rb) or sodium (Na) is deposited via sputtering.
However, even when the techniques as disclosed in Patent Documents 1 and 2 are used to prepare an X-ray sensitive phosphor, the emission efficiency when irradiated with X-rays have not fully been high enough. Specifically, in Patent Document 2, activators of CsI have been described, however, melting points of the activators have not been considered, and further improvement in the emission efficiency of the phosphor when irradiated is being studied.
Patent Document1 Examined Japanese Patent
Publication No. 54-35060
Patent Document 2 JP-A No. 2001-59899
(JP-A representing Japanese Patent Publication Open to Public Inspection)